Channel state information (CSI) is very important in a modern communications system, and can provide important information for resource scheduling, user transmission format determining, multi-user pairing, and even coordination between multiple cells. In a Long Term Evolution (LTE) system in the 3rd Generation Partnership Project (3GPP), CSI generally includes information such as a channel quality indicator (CQI), a precoding matrix indicator (PMI), and a rank indicator (RI). For a downlink (that is, a base station transmits data to user equipment) system, the user equipment may generally determine CSI according to a reference signal (RS, also referred to as a pilot) sent by the base station, and fed back the CSI to the base station.
Each reference signal generally corresponds to one antenna port. An LTE R8 system can support a configuration of a maximum of four antenna ports by using a cell-specific reference signal (CRS); LTE R10-R11 systems can support a configuration of a maximum of eight antenna ports by using a channel state information reference signal (CSI RS), where “LTE Rx” denotes “LTE of an xth release”.
To further improve a system capacity and system coverage, a configuration of more antenna ports is currently considered to be introduced. For example, an antenna configuration of 16, 32, 64, or more antenna ports may be introduced in an active antenna system (AAS) and Massive multiple-input multiple-output (MIMO). On the one hand, each antenna port generally corresponds to one reference signal, and each reference signal needs to occupy a time-frequency resource; therefore, a configuration of more antennas means more resource overheads. On the other hand, more antenna ports mean more corresponding channel measurement that needs to be performed by the user equipment and higher complexity of CSI measurement. Therefore, avoiding excessive resource overheads and excessively high complexity of CSI measurement performed by the user equipment while giving full play to a multi-antenna configuration is now an important problem that is urgent to be resolved.